Update Spherical kernel interface to use the new data organisation

- Divide the FSphericalCell inner data layout into two sub-types:
multipole_t and local_expansion_t. As the multipole and the local
expansion layout is the same, they share a common implementation.
Two class attribute are accessible through the `getMultipoledata` and
`getLocalExpansionData` methods.
- Change FMM operators signature to take advantage of the new
layout. The operators only take as parameters the data they may need
instead of the whole cell. For instance (simplified):
void M2M(CellClass* parent, CellClass** children);
becomes
void M2M(multipole_t* parent_m, symbolic_data_t* parent_s,
multipole_t** children_ms, symbolic_data_t* children_ss);